The first-pass effect of furosemide was investigated in rats. Furosemide intravenous solution (20 mg kg-1 Lasix), was administered via the jugular vein and the portal vein, orally, and instilled directly into the duodenum of rats. The first-pass effects of furosemide by lung, heart, and liver seemed to be negligible in rats. The absolute bioavailability of furosemide was 28·9 and 48·3% after oral and intraduodenal administration, respectively. Based on the gastrointestinal (GI) recovery study, 68·3 and 69·5% of furosemide were found to have disappeared mainly due to absorption and/or metabolism from rat GI tract after oral and intraduodenal administration, respectively.
The results indicate that gastrointestinal and intestinal first-pass effects of furosemide were approximately 40% (68·3–28·9%) and 20% (69·5–48·3%) of the dose, respectively.
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